The present invention relates to a process for producing aluminum support for a printing plate. In particular, roughening of the aluminum support which is present in an acid and/or salt electrolyte is effected by an direct current.
Aluminum plates (including aluminum alloy plates) are extensively used as supports for printing plates and particularly as supports for offset printing plates. In order to be used as supports for offset printing plates, such aluminum plates must exhibit a suitable degree of adhesion to photo-sensitive layers as well as good water retention. To this end, the surface of an aluminum plate is roughened to have a uniform and finely grained surface. Since this roughening or graining treatment significantly affects the printing performance and running characteristics (press life) of the offset printing plate produced from the grained aluminum plate, the success of the roughening treatment is a key factor in the production of printing plates.
Aluminum supports for printing plates are currently roughened using the a-c electrolytic etching method, with the current being an ordinary sinusoidal current or a special alternating wave (e.g. square wave ) current. Using graphite or some other suitable material as a counter-electrode, roughening of the aluminum plate is usually achieved with a single application of a-c current. A drawback of such technique, however, is that the depth of roughened surface by this method are generally small and insufficient to ensure good running characteristics. Therefore, various approaches have been proposed for making aluminum plates that have a grain characterized by a uniform and dense distribution of high depth-to-diameter ratio roughness and thus, are suitable for use as supports for printing plates.
The proposals made so far are based on optimizing parameters such as the ratio of electricity to be applied respectively to the anode and the cathode during electrolytic roughening with a-c current (Japanese Unexamined Patent Application (OPI) No. 65607/1979); the waveform of the voltage which is supplied from a power source (Japanese Unexamined Patent Application (OPI) No. 25381/1980); and combinations of variable currents to be applied per unit area (Japanese Unexamined Patent Application (OPI) No. 29699/1981).
Despite such efforts, however, the depth of roughened surface produced by these prior art techniques are not sufficiently large and have such uneven depth profiles so as to provide a complexly undulating pattern. Not surprisingly, therefore, offset printing plates made by using aluminum supportshaving such defective pits are far from being satisfactory in terms of both printing performance and running characteristics.
With a view to solving this problem, it was proposed in U.S. Pat. No. 4,482,434 to perform electrochemical roughening with a-c current having low frequencies, i.e., in the range of 1.5-15 Hz. However, the use of low-frequency a-c current in electrochemical roughening of an aluminum plate as proposed in U.S. Pat. No. 4,482,434 has its own disadvantages. First, a printing plate formed from an aluminum support that has been subjected to continuous electrochemical roughening by this method gives rise to uneven printing results due to the formation . of lateral defects that run perpendicularly to the direction in which the aluminum plate traveled. Second, the use of low-frequency a-c current is not adapted to commercial operations since the carbon electrode used in conventional electrochemical roughening undergoes rapid dissolution.
In addition to the above disadvantages, the a-c electrochemical roughening methods of the prior art typically require special power supply units which translates into increased equipment costs.